Reverse Transcriptase—Nucleoside Agents

The second class of agents comprises non-competitive inhibitors of reverse transcriptase. These agents are also referred to as non-nucleoside reverse transcriptase inhibitors (NNRTIs). Unlike NRTIs, NNRTIs do not require phosphorylation to be activated and do not compete with nucleoside triphosphates. The NNRTIs bind to a site on the viral reverse transcriptase that is close to but separate from the NRTI receptor site. This binding ultimately results in blockade of RNA- and DNA-dependent DNA... 

Derivatives of 439, a novel tricyclic ring system useful as lead structure for the design of novel non-nucleoside reverse transcriptase (NNRT) agents of therapeutic interest, has been synthesized <2002JHC81>. 

Hurwitz SJ, Schinazi RF (2002) Development of a pharmacodynamic model for HIV treatment with nucleoside reverse transcriptase and protease inhibitors. Antiviral Res 56 115-127 Hurwitz SJ, Tennant BC, Korba BE, Gerin JL, Schinazi RF (1998) Pharmacodynamics of (—)-beta-2, 3 -dideoxy-3 -thiacytidine in chronically virus-infected woodchucks compared to its pharmacodynamics in humans, Antimicrob Agents Chemother 42 2804-2809 Hurwitz SJ, Otto MJ, Schinazi RF (2005) Comparative pharmacokinetics of Racivir, (+/-)-beta-2, 3 -dideoxy-5-fluoro-3 -thiacytidine in rats, rabbits, dogs, monkeys and HIV-infected humans, Antivir Chem Chemother 16 117-127... 

There are currently six major antiretroviral drug families (Table 5). Nucleoside reverse transcriptase inhibitors (NRTI) are nucleoside analogs (discussed in more detail in chapter by De Clercq and Neyts, this volume) and were the first approved antiretroviral agents. They include drugs such as AZT, didanosine (ddl), stavudine (d4T), abacavir (ABC), and lamivudine (3TC), the latest used at doses of 300 mg daily as anti-HIV agent (lOOmg/day is the dosing approved for treatment of HBV... 

Data from Panel on Clinical Practices for the Treatment of HIV Infection. Guidelines for the Use of Antiretroviral Agents in HIV-Infected Adults and Adolescents. 2006, http/AMM/vAIDSinfo.NIH.gov Anderson PL, Kakuda TN, Lichtenstein KA. The cellular pharmacology of nucleoside- and nudeotide-andogue reverse-transcriptase inhibitors and its relationship to dinical toxdties. din Infect Dis 2004 38 743-753 and produd information for agents... 

Other applications include bioequivalent measurements of bromazepam, an anticonvulsant, in human plasma. The lower limit of quantitation (LLOQ) was 1 ng/mL (Gongalves et al. 2005). Kuhlenbeck et al. (2005) studied antitussive agents (dextromethorphan, dextrophan, and guaifenesin) in human plasma LLOQ values were 0.05, 0.05, and 5 ng/mL, respectively. Other compounds studied were nucleoside reverse transcriptase inhibitors, zidovudine (AZT) and lamivudine (3TC) (de Cassia et al. 2004) and stavudine (Raices et al. 2003) in human plasma, and paclitaxel, an anticancer agent, in human serum (Schellen et al. 2000). 

The development of antiretroviral therapy has been a major challenge since the discovery of the human inununodeficiency virus (HIV). Early successes with nucleoside and non-nucleoside reverse transcriptase (RT) inhibitors, as well as the development of protease inhibitors have facilitated, in recent years, a highly active antiretroviral therapy (HAART), where a combination of drugs is simultaneously administered. In spite of significant improvements in the morbidity and mortality of HIV-infected patients, the rapid appearance of resistant HIV-variants, as well as adverse effects and high cost of contemporary drugs necessitate the continuous development of independent classes of anti-HIV agents. ... 

The rapid spread of acquired immune deficiency syndrome (AIDS) has prompted numerous efforts to develop therapeutic agents against the human immunodeficiency virus type 1 (HIV-1) [2351. Efforts have focused on inhibition of the virally encoded reverse transcriptase (RT) enzyme, which is responsible for the conversion of retroviral RNA to proviral DNA. The nucleoside RT inhibitors 3 -azidothymidine (AZT) and dideoxyinosine (ddl) have proven to be clinically useful anti HIV-1 agents [236], but due to their lack of selectivity versus other DNA polymerases, these compounds are flawed by their inherent toxi-... 

Zidovudine (ZDV or AZT) is a nucleoside reverse transcriptase inhibitor (NRTI) and it was the first anti-HIV agent to be introduced. Other NRTIs include stavudine (d4T), lamivudine (3TC), didano-sine (ddl), abacavir (ABC) and zalcitabine (ddC). Recent additions to this class are emtricitabine (FTC) which has a molecular structure similar to 3TC and tenofovir (TDF) a nucleotide reverse transcriptase inhibitor. 

The replicative cycle of HIV presents many opportunities for the targeting of antiviral agents. The drugs in clinical use are classified as nucleoside reverse transcriptase inhibitors (NRTIs), nonnucleoside reverse transcriptase inhibitors (NNRTIs), nucleotide reverse transcriptase inhibitors (NTRTIs), and protease inhibitors (PI). 

Tenofovir disoproxil fumarate (Viread) is a prodrug of tenofovir, a phosphorylated adenosine nucleoside analogue, and is the only available agent of its class. It is converted by cellular enzymes to tenofovir diphosphate, which competes with deoxyadenosine triphosphate (dATP) for access to reverse transcriptase and causes chain termination following its incorporation. Tenofovir was approved as part of a combination therapy for HIV in adults who failed treatment with other regimens it appears to be effective against HIV strains that are resistant to NRTIs. The pharmacokinetic properties of tenofovir are provided in Table 51.2. 

The NNRTIs bind directly to HIV-1 reverse transcriptase (Figure 49-4), resulting in allosteric inhibition of RNA- and DNA-dependent DNA polymerase. The binding site of NNRTTs is near to but distinct from that of NRTTs. Unlike the NRTT agents, NNRTIs neither compete with nucleoside triphosphates nor require phosphorylation to be active. 

Birkus G, Hitchcock MJ, Cihlar T. Assessment of mitochondrial toxicity in human cells treated with tenofovir comparison with other nucleoside reverse transcriptase inhibitors. Antimicrob Agents Chemother 2002 46 716-23. 

Efavirenz (DMP 266) (1) is an effective non-nucleoside inhibitor of reverse transcriptase of the human immunodeficiency virus (HIV) recently registered by the US Food Drug Administration (FDA) for treatment of the acquired immunodeficiency syndrome (AIDS).1 2 3 Inhibition of HIV reverse transcriptase by nucleosides like azidothymidine (AZT) (2) is a proven therapy for delaying the progression to AIDS. However, the rapid viral mutation to resistant strains requires the development of new therapeutic agents. The recent development of both protease inhibitors and non-nucleoside reverse transcriptase inhibitors offers hope of effective treatment especially when coadministered. 

Resistance Viral isolates from patients who have undergone prolonged therapy with ddl contain reverse transcriptase with amino acid substitutions. Cross-resistance with the other nucleoside agents has not been reported. 

Treatment of human immunodeficiency virus (HIV) infections has been radically modified during the past two years by the introduction of powerful antiretroviral drugs, and by the development of methods to determine the viral burden in plasma. The introduction of HIV protease inhibitors and non-nucleoside reverse transcriptase inhibitors has made this breakthrough possible. However, the long-term therapeutic benefits and safety profiles of these agents are still being evaluated. 

---